Japanese Patent Application Laid-Open No. 2002-049076 discloses a driving apparatus incorporated in a light amount adjustment apparatus, which achieves an increased output and a small diametrical size with the rotation axis in the center. A breakdown perspective view of the driving apparatus is shown in FIG. 12, and a lateral cross-section is shown in FIG. 13.
Numeral 101 denotes a magnet comprising a permanent magnet, which is capable of rotating on a rotation center, and whose outer circumferential surface is divided into two in the circumferential direction so as to be magnetized to the south pole and the north pole alternately. Numeral 102 denotes a coil arranged in the axial direction of the magnet 101. Numeral 103 denotes a stator, which is excited by the coil 102, and which comprises a tooth-shaped outer magnetic pole portion 103a arranged at the end and an inner cylinder 103b. The stator 103 is arranged in a way that the outer magnetic pole portion 103a and the inner cylinder 103b respectively face the outer circumferential surface and the inner circumferential surface of the magnet 101. Numeral 104 denotes an auxiliary stator fixed to the inner cylinder 103b of the stator 103. Together with the inner cylinder 103b, the auxiliary stator 104 serves as an inner magnetic pole portion.
The magnet 101 comprises integratedly molded axes 101e and 101f so that the magnet 101 is held rotatable. The outer magnetic pole portion 103a of the stator 103 faces the outer circumferential surface of the magnet 101 with a gap. Also, the inner cylinder 103b of the stator 103 faces the inner circumferential surface of the magnet 101 with a gap.
In the above-described driving apparatus, the direction of electric application to the coil 102 is switched to change the polarity of the outer magnetic pole portion 103a, the inner cylinder 103b, and the auxiliary stator 104, thereby reciprocatively rotating the magnet 102. The reciprocative rotation of the magnet is controlled by running the driving pin 101d against a guide groove 105a provided on the bottom board 105.
In the driving apparatus, magnetic flux generated by energizing the coil flows from the outer magnetic pole portion to the opposite inner magnetic pole portion, or from the inner magnetic pole portion to the opposite outer magnetic pole portion, thus effectively acting on the magnet positioned between the outer magnetic pole portion and the inner magnetic pole portion. Furthermore, a resistance of the magnetic circuit configured by the outer magnetic pole portion and the inner magnetic pole portion is reduced as much as possible by making the distance between the outer magnetic pole portion and the inner magnetic pole portion equal to the total distance including the thickness of the cylindrical magnet, the gap between the magnet and the outer magnetic pole portion, and the gap between the magnet and the inner magnetic pole portion. The smaller the resistance of the magnetic circuit, the more the magnetic flux can be generated with a small amount of current, thus increasing an output of the driving apparatus.
However, in the construction disclosed in the Japanese Patent Application Laid-Open No. 2002-049076, since there is only one tooth-shaped outer magnetic pole portion 103a, the magnet 101 upon rotation is pulled toward a direction opposite to the outer magnetic pole portion 103a, causing poor balance and a significant loss. If the magnet is magnetized to four poles, two tooth-shaped outer magnetic pole portions can be provided and the rotational balance will improve. However, compared to the magnet magnetized to two poles, the rotation angle decreases by half.
Furthermore, the axial length of the driving apparatus is determined by the length of the coil 102, the length of the magnet 101, and the thickness of the stator 103. The external diameter of the driving apparatus is determined by the external diameter of the inner cylinder 103b, the radial thickness of the coil 102, and the thickness of the outer magnetic pole portion 103a. An increased number of turns in the coil will only be achieved by one of the following: reducing the length of the magnet 101, reducing the thickness of the outer magnetic pole portion 103a, increasing the length of the driving apparatus, or increasing the external diameter of the driving apparatus. Since reducing the length of the magnet causes a decreased output, it is limited. Since reducing the thickness of the outer magnetic pole portion 103a causes magnetic saturation and a decreased output, it is also limited. Therefore, increasing the external diameter of the driving apparatus has been the only way to realize a driving apparatus that achieves an increased output and a short axial length.
Moreover, a predetermined gap is necessary between the internal diameter of the magnet 101 and the auxiliary stator 104 opposite to the magnet 101. Controlling the gap at the time of manufacturing the apparatus causes an increased cost.
Further, the form of the stator calls for the inner cylinder 103b and the outer magnetic pole portion 103a. In the manufacturing process, these parts are difficult to construct integratedly. In a case where these parts are manufactured separately and integrated later, the number of parts will increase and the cost will increase.
Furthermore, reducing the diametrical thickness of the magnet 101 having a cylindrical shape for the purpose of reducing the distance between the outer magnetic pole portion 103a and the auxiliary stator 104 is difficult to achieve from the aspect of mechanical strength.